1. Field
Embodiments relate generally to articles such as light scattering inorganic substrates and methods for making light scattering inorganic substrates, and more particularly to light scattering inorganic substrates comprising monolayers and methods for making light scattering inorganic substrates comprising monolayers useful for, for example, photovoltaic cells.
2. Technical Background
For thin-film silicon photovoltaic solar cells, light must be effectively coupled into the silicon layer and subsequently trapped in the layer to provide sufficient path length for light absorption. A path length greater than the thickness of the silicon is especially advantageous at longer wavelengths where the silicon absorption length is typically tens to hundreds of microns. Light is typically incident from the side of the deposition substrate such that the substrate becomes a superstrate in the cell configuration. A typical tandem cell incorporating both amorphous and microcrystalline silicon typically has a substrate having a transparent electrode deposited thereon, a top cell of amorphous silicon, a bottom cell of microcrystalline silicon, and a back contact or counter electrode.
Amorphous silicon absorbs primarily in the visible portion of the spectrum below 700 nanometers (nm) while microcrystalline silicon absorbs similarly to bulk crystalline silicon with a gradual reduction in absorption extending to ˜1200 nm. Both types of material benefit from textured surfaces. Depending on the size scale of the texture, the texture performs light trapping and/or reduces Fresnel loss at the Si/substrate interface.
The transparent electrode (also known as transparent conductive oxide, TCO) is typically a film of fluorine doped-SnO2 or boron or aluminum doped-ZnO with a thickness on the order of 1 micron that is textured to scatter light into the amorphous Si and the microcrystalline Si. The primary measure of scattering is called “haze” and is defined as the ratio of light that is scattered >2.5 degrees out of a beam of light going into a sample and the total light transmitted through the sample. The scattering distribution function is not captured by this single parameter and large angle scattering is more beneficial for enhanced path length in the silicon compared with narrow angle scattering. Additional work on different types of scattering functions indicate that improved large angle scattering has a significant impact on cell performance.
The TCO surface is textured by various techniques. For SnO2, the texture is controlled by the parameters of the chemical vapor deposition (CVD) process used to deposit the films. An example of a textured SnO2 film is, for example, Asahi-U films produced by Asahi Glass Company. For ZnO, plasma treatment or wet etching is used to create the desired morphology after deposition.
Disadvantages with textured TCO technology can include one or more of the following: 1) texture roughness degrades the quality of the deposited silicon and creates electrical shorts such that the overall performance of the solar cell is degraded; 2) texture optimization is limited both by the textures available from the deposition or etching process and the decrease in transmission associated with a thicker TCO layer; and 3) plasma treatment or wet etching to create texture adds cost in the case of ZnO.
Another approach to the light-trapping needs for thin film silicon solar cells is texturing of the substrate beneath the TCO and/or the silicon prior to silicon deposition, rather than texture a deposited film. In some conventional thin film silicon solar cells, vias are used instead of a TCO to make contacts at the bottom of the Si that is in contact with the substrate. The texturing in some conventional thin film silicon solar cells consist of SiO2 particles in a binder matrix deposited on a planar glass substrate. This type of texturing is typically done using a sol-gel type process where the particles are suspended in liquid, the substrate is drawn through the liquid, and subsequently sintered. The beads remain spherical in shape and are held in place by the sintered gel.
Many additional methods have been explored for creating a textured surface prior to TCO deposition. These methods include sandblasting, polystyrene microsphere deposition and etching, and chemical etching. These methods related to textured surfaces can be limited in terms of the types of surface textures that can be created.
Light trapping is also beneficial for bulk crystalline Si solar cells having a Si thickness less than about 100 microns. At this thickness, there is insufficient thickness to effectively absorb all the solar radiation in a single or double pass (with a reflecting back contact). Therefore, cover glasses with large scale geometric structures have been developed to enhance the light trapping. For example, an EVA (ethyl-vinyl acetate) encapsulant material is located between the cover glass and the silicon. An example of such cover glasses are the Albarino® family of products from Saint-Gobain Glass. A rolling process is typically used to form this large-scale structure.
Disadvantages with the textured glass superstrate approach can include one or more of the following: 1) sol-gel chemistry and associated processing is required to provide binding of glass microspheres to the substrate; 2) the process creates textured surfaces on both sides of the glass substrate; 3) additional costs associated with silica microspheres and sol-gel materials; and 4) problems of film adhesion and/or creation of cracks in the silicon film.
For traffic safety, 3M manufacturers a wide range of products that incorporate retroreflector technology. One technology type is glass beads which are mirrored on the backside. To process these in large quantities, they form a monolayer of glass beads on an adhesive layer. The beads are typically on the order of tens of microns in size. The early patents on this technology suggest that the process of forming a monolayer using adhesive works for sizes of a few microns up to about 125 microns. In addition, oleophobic coatings are applied to the glass beads to control the amount they sink into the adhesive coating.
It would be advantageous to have a method for making a light scattering inorganic substrate wherein a monolayer of particles could be formed on the substrate. Further, it would be advantageous for the coating method to be adaptable for large substrates and adaptable to a continuous coating process.